The letter u before any unit means micro. Micro is a unit prefix used in the metric system which actually means a factor of 10^-6 (one millionth).
Now, 10^-6 is equivalent to a movement of 6 decimal places from left to right. So to write the number 0.0000078 s into a number which has a unit of us, therefore we move the decimal place by 6 to the right, giving us:
7.8 us
Another way to do this is to multiply the number by 1 million (1,000,000), so:
0.0000078 * 1,000,000
Using the calculator to solve, we get the value:
7.8 us